The invention relates to a high-flux neutron generator, comprising a target to be struck by a hydrogen isotope ion beam, and which is formed by a structure comprising a metallic layer having a high hydrogen absorption coefficient which is deposited on a carrier layer which is made of a metal having a high thermal conductivity coefficient and a low degree of volatilization.
Generators of this kind are used for example for the examination of matter by means of fast neutrons, thermal neutrons, epithermal neutrons or cold neutrons.
The neutrons are generated by reactions between nuclei of heavy hydrogen isotopes: deuterium and tritium. These reactions take place by subjecting a target, containing deuterium and tritium, to bombardement by a beam of deuterium ions and tritium ions which are accelerated under the influence of a high potential difference. The deuterium ions and tritium ions are formed in an ion source in which a gaseous mixture of deuterium and tritium is ionized. The collision between a deuterium nucleus and a tritium nucleus produces a neutron with a binding energy of 14 MeV, and a -.alpha. particle with a binding energy of 3.6 MeV.
In order to obtain the maximum reaction yield, the target nucleus density should be as high as possible. A contemporary means of achieving such targets with hydrogen isotopes consists in binding of the nuclei in the crystal lattice of a hydrogenizable material.
Among these materials titanium is often used because of its lower stopping power, resulting in a higher neutron yield. These materials have the drawback, however, that they have insufficient mechanical strength when the hydrogen concentration is high and the material is provided in a "thick layer" (a splitting phenomenon causing the dispersion of the metallic particles which is detrimental to the voltage holdoff, of ion beam acceleration devices.
Consequently, these materials must be used in thin layers deposited on a carrier or substrate which must have a low absorption and diffusion coefficient for hydrogen, a suitable thermal conductivity to enable removal of the dissipated energy and a high corrosion resistance in respect of the cooling liquid. For example, a copper carrier partly satisfies these criteria but has a high sputtering coefficient. A target having suitable mechanical strength is difficult to realize by means of such a carrier, because the linear expansion coefficient of titanium deviates substantially from that of copper. Moreover, in the case of a beam with a non-uniform energy density the service life of the target is very short because of the fact, that after erosion of the titanium layer at areas of high density impact of the ion beam, the copper of the support is quickly sputtered on the surface of the surrounding titanium, thus substantially slowing the ion energy and hence the neutron yield; moreover, the carrier layer there is also pierced.
One way of avoiding this phenomenon consists in the insertion of an intermediate layer of a material such as molybdenum, having a higher ion erosion resistance and being less permeable to hydrogen and its isotopes, between the carrier layer and the metallic surface layer absorbing the hydrogen. Thus, the hydrogen ion concentration of the surface layer increases rapidly until a state of equilibrium is established in which the amount of hydrogen penetrating said surface layer is equal to that emanating therefrom by diffusion. The maximum concentration of tritium atoms is thus obtained in the thin titanium layer, so that the neutron yield is highest.
In French Patent Specification No. 7924106 (issue No. 2-438-953) the deposition of a second intermediate layer of a material, teaches that such as vanadium whose linear expansion coefficient is between that of the carrier layer and that of the first intermediate layer offers better adherence of the contacting surfaces.
The successive improvements of the target in the cited embodiments aim to prolong the service life of the target by retarding the erosion of the substrate by the ion bombardement. It is to be noted that the beam is formed by an equimolecular mixture of deuterium and tritium so that the ions extracted from the source and implanted in the target after acceleration do not lead to the depletion of the target nuclei for the benefit of the beam nuclei.
At this stage, the ion implantation of the beam takes place in layers of carrier materials whose stopping power, being much higher than that of the active layer, causes a substantial drop of the neutron emission, leading to the end of the service life of the tube.
It is the object of the invention to provide a neutron generator which comprises a target for a hydrogen isotropic ion beam, the service life of target exposed to bombardement by a high-intensity ion beam being longer than the service life of the targets of known neutron generators.
The neutron generator of the kind set forth in accordance with the invention is characterized in that the active layer having a high hydrogen absorption coefficient is formed by a stack of identical layers which are isolated from one another by a diffusion barrier, the thickness of the layers having a high absorption coefficient being equal, for example to the penetration depth of the deuterium ions striking the target. This thickness depends upon the acceleration voltage=for example about 4 microns at 250 kV.
Thus, the hydrogenation of the deep layers takes place only step-wise during the piercing of the diffusion barriers under the influence of erosion due to the bombardement. The service life of known targets, comprising only a single active layer, can thus be multiplied by the number of active layers superposed in the target in accordance with the invention.
Moreover, because the diffusion of the tritium is limited to the thickness of one layer, the concentration of the target nuclei is not reduced beyond the penetration zone of the beam; this offers the dual advantage that the impregnation of the target is accelerated and that the neutron yield is improved.
Another advantage consists in the reduction of the total quantity of the mixture of deuterium and tritium required for the operation of the generator, notably in as far as it concerns the amount of tritium which is progressively decomposed into He.sub.3, thus increasing the residual pressure in the tube in a correlative fashion.